The present invention relates to a method and an apparatus for forming an electrophotographic image for printers, copying machines, and facsimile apparatus in which toner is deposited on an electrostatic latent image formed on an image carrying member to develop the image.
Methods of developing an electrostatic latent image on an image carrying member by applying a development bias having a DC component and an AC component superposed on one another to a developer carrying member is well known by, for example, JP-A-58-32377 and JP-A4-56976.
FIG. 1 is a diagram for explaining a method of applying a development bias of the related art, and it shows a waveform of a development bias in a case wherein an image carrying member is charged at −600 V and development is performed using a negatively charged toner with the image portion charged at a potential of −70 V. One period of the development bias has a time t1 on a development restraining side of the period during which the toner deposited on the image carrying member is separated and peeled off from the member toward a developer carrying member and a time t2 on a development promoting side of the period during which the toner is transferred from the developer carrying member to the image carrying member. According to the related method, the toner is repeatedly deposited on and separated from the image carrying member due to an oscillating electric field formed at the portion to be developed, and the toner is finally deposited on the electrostatic latent image and remains thereon to complete development as a result of attenuation of the oscillating electric field attributable to an increase in the interval between the image carrying member and the developer carrying member.
FIG. 2 shows γ-characteristics of gradation in a case wherein the development bias in FIG. 1 is set at frequencies of 1 kHz and 3 kHz, an amplitude Vpp of 1600 V, and a DC component average potential Vavg of −200 V. FIG. 3 shows γ-characteristics of a contrast potential (an image portion potential Vs−Vavg) at a duty ratio ACDuty of 40%. FIGS. 4 and 5 show contrast potential γ-characteristics and gradation γ-characteristics, respectively, at a frequency of 3 kHz and duty ratios ACDuty of 40%, 60%, and 80%. A duty ratio ACDuty is defined as t1/(t1+t2)×100(%).
As shown in FIG. 2, at a frequency as high as 3 kHz, the gradation γ-characteristics are in the form of the character S rather than being linear, which results in a low density in a highlight and missing gradations in a shadow. At a frequency as low as 1 kHz, the gradation γ-characteristics are linear, and preferable gradation is exhibited in that a highlight has a high density and no gradation is lost in a shadow. However, a problem arises as shown in FIG. 3 in that there are many fogs on the background at a non-image potential at the low frequency of 1 kHz although there are less fogs on the background at the rion-mage potential at the high frequency of 3 kHz.
In JP-A-56976, a solution to this problem is sought through appropriate selection of a ratio between the product of voltage and time on the development promoting side and the product of voltage and time on the restraining side. However, when the duty ratio is varied for a single frequency, the linearity of gradation γ-characteristics cannot be improved as shown in FIG. 5, although there is no increase in fogs on the background as shown in FIG. 4. Thus, there is a problem in that it is difficult to increase the density of a highlight.
That is, it has been difficult to achieve both of (1) gradation γ-characteristics with high reproducibility of a highlight and high linearity and (2) formation of an image without deposition of toner on a non-image portion by simply changing the frequency or duty ratio.